Most of John Cage’s music comes preloaded with a preposterous philosophical challenge of some kind. Cage the playful instigator, the experimental trickster, must have loved the sound — the actual sound — of people arguing over what is and what is not music. It is hard not to view the intellectual hubbub that attends his work — the debates, the defenses, the visceral disgust, the dense rationales — as a movement of the work itself, part of the tune, as it were: words like rain on the roof.